Orifice plate flow meters having a single centered opening in a plate constriction that is disposed diametrically within a fluid-carrying pipe, with differential pressure measurement means on the upstream and downstream sides of the constriction, have been in existence for a long period of time. While the accuracy of such devices is good for long runs of pipe, orifice plate flow meters suffer the disadvantage of poor performance when placed in short pipe runs that follow a flow disturbance created by upstream apparatus, such as an elbow, expander, reducer, valve or other discontinuity. For measurement accuracy with an orifice plate flow meter, a long straight run of pipe (in excess of ten diameters in some instances) upstream of the flow meter is required in order to present a fully developed symmetrical velocity profile to the orifice plate, with the highest fluid velocity occurring coaxially with the center of the orifice plate constriction. When an upstream pipe fitting or other device skews the velocity profile, the pressures measured at the orifice flow meter pressure taps is in error.
To reduce the asymmetry of the velocity profile created by an upstream fitting, the commonly used apparatus is a flow straightener, of the type disclosed in U.S. Pat. No. 5,596,152 or apparatus similar to the flow conditioner described in U.S. Pat. No. 3,733,898. A more complex apparatus, employing a plurality of elongated openings arranged in a predetermined pattern in a transversely disposed plate within the fluid-conducting pipe, together with a computer to deal with the necessary algorithms, is disclosed in U.S. Pat. No. 5,295,397. Yet another device for reducing the adverse measuring effects of a distorted velocity profile is known as the piezometer ring. This appliance may surround the orifice on both sides of the orifice plate and, by means of a plurality of circumferentially disposed pressure sensing ports, averages pressures around the upstream and downstream sides of the orifice. Although not shown with an orifice plate flow meter, a piezometer type of averaging sensor is described generally in U.S. Pat. No. 5,279,155.
Flow straighteners, conditioners, computers and piezometers are moderately effective to properly condition the velocity profile for introduction to an orifice plate flowmeter, or average the asymmetric velocity of the flow, but have the disadvantage of adding separate and additional components to the process piping with the attendant initial cost, pressure drop in the fluid, and increased maintenance requirements.
Accordingly, the primary object of the present invention is to provide a primary flow element that achieves the accuracy benefits of the orifice plate type of flow meter, but is not restricted to long runs of upstream piping prior to the flow meter's positioning in the pipe.
A corresponding objective of the invention is to provide a primary flow element where the means for interrupting the fluid flow is a differential pressure orifice plate that achieves an averaging of the differential pressures across the plate, despite velocity profile distortion of the fluid presented to the primary flow element, and without the added piezometer and its computer, flow straightening or flow conditioning apparatus.
Other and further objects, features and advantages of the invention will become apparent from the following description of embodiments of the invention, taken in conjunction with the accompanying drawings.